This relates to RFID systems and, more particularly, to systems that provide real time RFID tag location information.
One of the shortcomings of prior art RFID base stations is that they are not able to relatively precisely determine the locations of the RFID tags from which signals are received. One current-day approach for addressing this shortcoming is to employ a signal which the tag periodically transmits, in conjunction with a network of base stations receive the tag's transmission, and through triangulation that is based on the information derived from the strengths of the received signal, or from time-of-flight information (i.e., differences in time of arrival) the tag's location is approximated. Since the absolute strength of the tag's transmission is not known, the triangulation must operate on the relative transmitted powers, and the resultant triangulation calculations are quite difficult, requiring numerical methods to obtain a solution.
U.S. Pat. No. 7,061,428 discloses a method for estimating the distance of the tag from the base station, but this method cannot tell from which direction the signal came. With distances known, the location's calculation is much simpler, but still complex.
Thus, base stations that are located in rooms of a building, such as a hospital, are unable to identify whether a received signal is from a tag that is in the same room, or in an adjacent room on the same floor, or on a floor above or below the base station without extensive processing. The difficulty stems from the fact that neither the base stations nor the tags employ directional antennas and, at least in the U.S., many buildings are constructed with walls that are made of materials that are substantially transparent to RF radiation.
Some artisans tried to solve the localization problem using IR technology, but IR has its own significant shortcomings; the major one being that it performs poorly without a clear line of sight between the tag and the base station. As a result, tags sometime use both IR and RF technologies to try and solve the problem, but this “fusion” technology does not work much better, since once the IR signals are not received by the base station, one is left with the aforementioned problem of RFID tags.